The United Nations is preparing to evacuate all non-essential staff from Syria, citing the "prevailing security situation" amid growing fears in Washington that the beleaguered regime is considering using chemical weapons.

The European Union also announced it was cutting back its activities in the country and on a fast-moving day of diplomatic and military action the Syrian government's foreign ministry spokesman, Jihad Makdissi, was reported to have defected.

The UN's undersecretary for safety and security, Gregory Starr, announced that the organisation had also cancelled all missions to Syria from abroad and suspended its activities inside the war-ravaged country. The decision marks the final step before a full-scale evacuation, a move that has not been ordered at any point during Syria's steady descent into chaos over the past 20 months.

Up to 25 of about 100 foreign staff may leave this week, it said, adding that more armoured vehicles were needed after attacks in recent weeks on humanitarian aid convoys and the hijacking of goods or vehicles. Some convoys were caught in crossfire between government and rebel forces, including an incident near the airport in which two staff were injured, it said.

The UN deploys more than 1,000 national and international staff in Syria, but movement and communications have become more difficult due to intensified fighting near the capital and a 48-hour internet blackout last week, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.

While there has been intense fighting on the ground, particularly in Damascus, in the past few days, there is also growing international concern that the Syrian regime is contemplating using chemical weapons. Syria on Monday denied it planned to use its chemical weapons stockpile, after reports that the US had observed officials moving some components of the programme. "Syria has stressed repeatedly that it will not use these types of weapons, if they were available, under any circumstances against its people," the foreign ministry said.

The US secretary of state, Hillary Clinton, who had warned that Washington would take action if Syria used chemical weapons, said: "I am not going to telegraph any specifics what we do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people, but suffice to say we are certainly planning to take action if that eventuality were to occur."

Late on Monday, White House spokesman Jay Carney echoed Clinton's words. "We are concerned that in an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people," he said. "And as the president has said, any use or proliferation of chemical weapons by the Syrian regime would cross a red line for the United States."

Turkish officials told the Guardian they have credible evidence that if the Syrian government's aerial bombardment against opposition-held areas fails to hold the rebels back, Bashar al-Assad's regime may resort to missiles and chemical weapons in a desperate bid to survive.

US and allied intelligence has also detected Syrian movement of chemical weapons components in recent days, a senior US defence official told the Associated Press. The source said intelligence officials had detected activity around more than one of Syria's chemical weapons sites in the last week.

The Assad regime saw the first high-profile departure from its ranks in recent months on Monday with the apparent defection of Makdissi. He has left Damascus, reportedly for London.

He had been a forceful defender of the regime since the earliest days of insurrection. However, the once prolific user of Twitter had not posted since late October and had been almost invisible in Syrian and foreign media for weeks.

The Beirut-based Hezbollah TV station al-Manar, claimed he had been removed because he was at odds with regime views. Diplomatic sources said he had defected.

Fighting continued in Damascus for a fifth day on Monday, with the international airport again receiving no flights, although unlike late last week it notionally remained open. Regime forces clashed with rebels nearby throughout the day.

Intense outgoing rocket fire could be heard from behind the Kass Youn mountain, on the city's eastern fringe. The rockets appeared to be aimed at rebel strongholds on the rural outskirts of the capital, particularly Darraya, which has remained a staunch opposition hub despite months of security sweeps by regime troops and bombing by jets.

The fighting in the capital is the most significant challenge to the power base of the Assad regime since mid-July, when rebel groups launched co-ordinated raids. That assault was put down by loyalist army divisions within a fortnight. The latest attack comes after steady gains in rebel capability in the north and near Damascus.

The EU said last night it was cutting back activity in Damascus. "The EU delegation has decided to reduce activities in Damascus to a minimum level due to the current security conditions," a spokesman for EU foreign policy chief Lady Ashton said.

A Lebanese MP confirmed on Monday that phone recordings relating to weapons transfers from Turkey to Syrian rebels, published in Lebanese media outlets, were of his voice.

Orkab Sakr, who is aligned to the Future political bloc of the exiled opposition leader Saad Hariri, confirmed that he had organised the transfer of weapons to Syria.

However, he said his activities for Hariri, who acts as the de facto head of the Sunni community in Syria as well as Lebanon, were limited to humanitarian missions.Save the Children claimed on Monday that an aid shortfall of more than $200m was hindering relief efforts as winter sets in. More than 400,000 refugees are thought to be living in temporary accommodation outside Syria, and many more are expected to flee.